Wittgenstein
The philosopher of language Ludwig Wittgenstein (1889 - 1951) argued that there is no set of concepts that capture reality thanks to their unambiguous "meanings"; deep down, meaning rides on game-like social practices. (Image: "Wittgenstein," by Mitch Francis.)

King
Martin Luther King, Jr.'s (1929 - 1968) understanding of his activism was subtly philosophical - and more than a little indebted to Socrates. (Image: "MLK," by Mitch Francis.)

Sartre
French philosopher Jean-Paul Sartre (1905 - 1980) defended a form of radical personal freedom which is especially associated with his "existentialism." (Image: "Sartre," by Mitch Francis.)